Effective bug reporting

Reporting bugs effectively is an important skill for any Fedora user or developer.

Narrowing down the possible causes of the bug and providing the right information in the bug report allows a bug to be resolved quickly. Filing a bug report with little useful information can mean that your bug lays unresolved, possibly until it is closed automatically when the distribution version reaches "end of life".

You can also check what virtualization capabilities are available on your machine by running:

$> virsh capabilities

Guest Configuration

When filing a bug related to problems seen in the guest, include full details on the guest configuration including CPU architecture, RAM size, devices etc. This is most easily done by including the output of virsh dumpxml MyGuest or, in the case of qemu, the full qemu command line.

Virt Manager

Virt Manager stores a logfile in ~/.virt-manager/virt-manager.log.

Examine the log file and include any pieces that look like they might be useful in the bug report. If in doubt, attach the whole file to the bug.

You can also run virt-manager from the command line using virt-manager --no-fork and check whether any relevant messages were printed there.

virt-install

virt-install stores a log file in ~/.virtinst/virt-install.log.

Run virt-install using the --debug option to get detailed debug spew.

In order to gain access to a serial console during the install, you can use -x "console=ttyS0". Using a serial console combined with a VNC install can be very useful for debugging e.g. --nographics -x "console=ttyS0 vnc"

libvirt

Any program using libvirt can be debugged using the LIBVIRT_DEBUG=1 environment variable e.g.

xen

If a guest is crashing you can obtain a stack trace by doing the following:

Set "on_crash=preserve" in your domain config

Copy the guest kernel's System.map to the host

Once the guest has crashed, run /usr/lib/xen/bin/xenctx -s System.map <domid>

General Tips

System Log Files

Always look in dmesg, /var/log/messages etc. for any useful information.

strace

strace can often shed light on a bug - e.g. if you run virt-manager, or libvirtd or qemu-kvm under strace you can see what files they accessed, what commands they executed, what system calls they invoked etc.:

$> strace -ttt -f libvirtd

If the program in question is already running, you can attach to it using strace -p.

gdb

gdb can often be useful to trace the execution of a program. However, in order to get useable information, you will need to install "debuginfo" packages. See the StackTraces page for more information.

SELinux

If you see "AVC denied" or "setroubleshoot" messages in /var/log/messages, your bug might be caused by an SELinux policy issue. Try temporarily putting SELinux into "permissive" mode with:

$> setenforce 0

If this makes your bug go away that doesn't mean your bug is fixed, it just narrows down the cause! You should include the AVC details from ausearch -m AVC -ts recent in the bug report, or if the message includes a sealert -l command then include the details printed by the command.

One common cause of SELinux problems is mis-labelled files. Try:

$> restorecon /path/to/file/in/selinux/message

If you are installing using an ISO on an NFS mount, you need to ensure that it is mounted using the virt_content_t label:

$> mount -o context="system_u:object_r:virt_content_t:s0" ...

If you are using libvirt storage pools, like nfs, or USB pass-through, you might want to check, or toggle one of the following SELinux booleans: virt_use_comm, virt_use_fusefs, virt_use_nfs, virt_use_samba, virt_use_usb.

Troubleshooting

Permission issues

Prior to Fedora 11/libvirt 0.6.1, all virtual machines run through libvirt were run as root, giving full administrator capabilities. While this simplified VM management, it was not very security conscious: a compromised virtual machine could possibly have administrator privileges on the host machine.

In Fedora 11/libvirt-0.6.1, security started to improve with the addition of svirt. In a nutshell, libvirt attempts to automatically apply selinux labels to every file a VM needs to use, like disk images. If a VM tries to open a file that libvirt didn't label, permission will be denied.

Fedora 12 saw things improve even more. As of libvirt-0.6.5, VMs were now launched with reduced process capabilities. This prevented the VM from doing things like altering host network configuration (something it shouldn't typically need to do). And as of libvirt-0.7.0, the VM emulator process was no longer run as 'root' by default, instead being run as an unprivleged 'qemu' user.

While all these changes are great for security, they broke previously working setups which depended on the relaxed VM permissions. Most issues have work arounds that come at the expense of security. Over time, many of these issues should be made to 'just work', but we aren't there yet.

Changing the QEMU/KVM process user

Changing the QEMU/KVM process user has security implications.

To change the user that libvirt will run the QEMU/KVM process as, edit /etc/libvirt/qemu.conf and uncomment and change the user= and group= fields. For example, if wanting to run KVM as the user 'foobar', you would set the fields to

...
user='foobar'
group='foobar'
...

Then restart libvirtd with

service libvirtd restart

Changing SVirt/Selinux configuration

Changing the SVirt/SELinux settings may have security implications.

SVirt can be disabled for the libvirt QEMU driver by editting /etc/libvirt/qemu.conf, uncommenting and setting

security_driver='none'

Then restart libvirtd with

service libvirtd restart

Changing QEMU/KVM process capabilities

Changing the this setting has security implications.

Libvirt by default launches QEMU/KVM guests with reduced process capabilities. To disable this feature, edit /etc/libvirt/qemu.conf, uncomment and set

clear_emulator_capabilities=0

Then restart libvirtd with

service libvirtd restart

KVM performance issues

Often times, VM slowness is caused because the VM is using plain QEMU and not KVM.

Ensuring system is KVM capable

Verify that the KVM kernel modules are properly loaded:

$ lsmod | grep kvm
kvm
kvm_intel

If that command did not list kvm_intel or kvm_amd, KVM is not properly configured. See this KVM wiki page to ensure your hardware supports virtualization extensions. If it doesn't, you cannot use KVM acceleration, only plain QEMU is an option.

If your hardware does support virtualization extensions, try to reload the kernel modules with:

su -c 'bash /etc/sysconfig/modules/kvm.modules'

Retry the above lsmod command and see if you get the desired output. If not, or if the kvm.modules command produces an error, check the output of:

dmesg | grep -i kvm

If you see 'KVM: disabled by BIOS', please see the relevant KVM wiki page
Any other error message is probably a bug, and should be reported.

If all that works out fine, you want to make your that your VMs are actually using KVM

Is My Guest Using KVM?

Often people are unsure whether their qemu guest is actually using hardware virtualization via KVM.

Serial console access for troubleshooting and management

Serial console access is useful for debugging kernel crashes and remote management can be very helpful.

Fully-virtualized guest OS will automatically have a serial console configured, but the guest kernel will not be configured to use this out of the box. To enable the guest console in a Linux fully-virt guest, edit the /etc/grub.conf in the guest and add 'console=tty0 console=ttyS0'. This ensures that all kernel messages get sent to the serial console, and the regular graphical console. The serial console can then be access in same way as paravirt guests:

su -c "virsh console <domain name>"

Alternatively, the graphical virt-manager program can display the serial console. Simply display the 'console' or 'details' window for the guest & select 'View -> Serial console' from the menu bar. virt-manager may need to be run as root to have sufficient privileges to access the serial console.

Graphical console access

In order to get a graphical console on your guest you can either use 'virt-manager' and select the console icon for the guest, or you can use the 'virt-viewer' tool to just directly connect to the console:

virt-viewer guestname

Accessing data on guest disk images

If the guest image might be live, you must only use read-only access, otherwise you risk corrupting the disk image.

It is always safe to use guestfish --ro

The guestfish package allows you to use a simple shell interface to manipulate guest disk images without needing to run the guest:

See 'man guestfish' and the libguestfs website for information and examples. guestfish can also be scripted to change a group of guest disk images in a row.

Known issues

Audio output

Audio has always been difficult to get working with libvirt, but the recent security changes have actually provided the mechanisms to make it work. The primary problem is that the VM is not sending sound output to your user's pulseaudio session. There may be a pulseaudio option to work around this issue, but I've managed to make it work with:

PCI device assignment

Libvirt's default behavior of dropping QEMU/KVM process capabilities prevents PCI device assignment from working correctly. See bug 573850 for more info. I only managed to get this working with the following steps: